1. Field of the Invention
The present invention relates to a radiation image detecting device that performs automatic exposure control and a control method thereof, and a radiation imaging system.
2. Description Related to the Prior Art
In a medical field, a radiation imaging system, for example, an X-ray imaging system using X-rays is widely known. The X-ray imaging system is constituted of an X-ray generating device having an X-ray source for emitting X-rays and an X-ray image detecting device for detecting an X-ray image. A type of X-ray image detecting device using a flat panel detector (FPD), which has a matrix of pixels each for accumulating electric charge in accordance with the amount of the X-rays incident thereon, as a detection panel becomes widespread. The FPD accumulates the electric charge on a pixel-by-pixel basis, and converts the accumulated electric charge into a voltage signal in its signal processing circuit to output the X-ray image of an object as digital image data.
In X-ray imaging, it is required to apply just the right amount of radiation dose to the object (patient) without excess and deficiency. Thus, the X-ray imaging system performs automatic exposure control (AEC) by which a radiation dose is measured using the FPD, and X-ray emission is automatically stopped when the radiation dose has reached a predetermined value. In the AEC, a region of space used for measuring the radiation dose is called a measurement field. The pixels of the FPD included in the measurement field are used for measuring the radiation dose. Therefore, the choice of the measurement field translates into the choice of the pixels to be used for measuring the radiation dose, in other words, the setting of a measurement area in the FPD. Generally speaking, with the aim of having optimal image density at a region of interest (ROI) being the most notable in diagnosis, a region of radiation passed through the ROI is chosen as the measurement field. The measurement field is determined in accordance with a body part to be imaged. Taking chest imaging as an example, a region of radiation passed through the lungs is designated as the measurement field.
Japanese Patent Laid-Open Publication No. 2002-000590 discloses an X-ray image detecting device in which when the body part such as chest or abdomen is designated in a body part setter, a pixel selector chooses the pixels of the FPD and the measurement field is automatically set. Japanese Patent Laid-Open Publication No. 2011-010870 discloses an X-ray imaging device in which an imaging surface of the FPD is partitioned into a plurality of areas (measurement areas). In the X-ray imaging device, a current integrating means integrates electric current flowing through the pixels of each area, and a measurement field determining means automatically determines the measurement field by comparing the integrated values. To be more specific, the area having the lowest integrated value is determined as the measurement field, because if an area receiving the least radiation dose has predetermined image density, the other areas necessarily have image quality higher than predetermined quality.
According to the Japanese Patent Laid-Open Publication No. 2002-000590, the measurement field is determined in accordance with the designated body part. In order to appropriately perform the AEC, it is required that the measurement field and the ROI of the patient are accurately positioned so as to coincide with each other.
Especially in oblique imaging in which the X-rays are applied to the patient in an oblique direction, since the measurement field is skewed with respect to the FPD, it is difficult to achieve precise positioning among the X-ray source, the FPD, and the ROI. Inappropriate positioning allows the X-rays that have not passed through the ROI to enter the measurement field. This causes excess or deficiency of an X-ray dose, and deteriorates the image quality of the ROI.
The Japanese Patent Laid-Open Publication No. 2011-010870 does not bring about the above problem, because the measurement field is automatically set up based on the distribution of the actually measured X-ray dose. However, the image quality of the ROI could vary depending on the position of the measurement field automatically set up in imaging. Also, an automatic setting process of the measurement field requires much time, as compared with a manual setting process by which the position of the measurement field is determined in accordance with the body part, and hence elongates X-ray emission time. The longer the X-ray emission time, the more the X-ray exposure of the patient increases and the likelier a blur occurs in the X-ray image by the effect of a body motion of the patient.